Searching cosmic dust

January 10, 1998|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON -- Astronomers say they have detected the glow of intergalactic dust heated by all the stars and galaxies that have ever shined, and found that half to two-thirds of those objects have somehow since disappeared.

They may be too faint, too far away or too exotic to be seen. Or they may just be lost in the dust clouds they've warmed.

Whatever the answer, the challenge for astronomers and theoreticians now is to launch new instruments to find the missing stars and galaxies, or to come up with new theories to account for what happened to them as the universe evolved.

"It is an important result," said Dr. David Spergel, a Princeton University cosmologist not connected with the research.

He said some astronomers, wowed by the Hubble Space Telescope's 1996 pictures of young galaxies crowding the most distant reaches of space and time, had thought science had finally revealed how many galaxies the universe has produced, he said.

"Now, nature is telling us, no, it hasn't," Spergel said. "Much of star formation is hidden from us. This places a basic energy constraint on the universe" that all subsequent researchers will have to consider.

The discovery was made by a large team of scientists using NASA's Cosmic Background Explorer (COBE) satellite. It was reported yesterday to astronomers at the winter meeting of the American Astronomical Society.

The team's findings were a long time coming.

The scientific team first met to plan its work 23 years ago. It was not until 1989 that the COBE spacecraft was launched. Gathering the data with COBE's Diffuse Infrared Background Experiment (DIRBE) instrument took from December 1989 until September 1990, and required 200 million observations in 10 wavelengths.

Scientists needed more than eight years to process, verify and analyze their measurements.

Intergalactic "dust" is anything outside galaxies that is solid -- from actual dust to mountain-sized boulders. This dust absorbs energy in the form of starlight, then re-emits it in infrared wavelengths. Infrared radiation is what you feel on the bottom of your feet when you walk across a hot sandy beach in summer.

Intergalactic dust is scarce by household standards -- on average, barely two microscopic particles in a volume the size of a hotel ballroom. But over the vast distances of space, dust NTC clouds can block starlight, and their glow can reveal stellar influences and intricate structures that would otherwise be invisible.

"We infrared astronomers love dust," said DIRBE team member Eli Dwek of the Goddard Space Flight Center in Greenbelt. The question has always been, "what fraction of stars are hidden by dust?"

Theoreticians had long expected to find the glow from warmed-over intergalactic dust coming in uniformly from all points in the sky.

If it could be measured, they said, cosmologists could use the data to calculate how many stars and galaxies must have shined on it since the first dust formed. That would have been 300,000 to 500 million years after the Big Bang, when the nuclear furnaces in the first stars began producing the "heavy" elements needed to form dust.

The Big Bang is the colossal explosion 12 billion to 15 billion years ago that most scientists believe marked the beginning of all space, matter and time.

The infrared glow is "not very bright," said DIRBE investigator Michael G. Hauser of the Space Telescope Science Institute in Baltimore. It's comparable to the illumination on a white shirt cardboard in downtown Washington from a 100-watt light bulb in the Rockville suburbs.

Sorting that feeble glow from the relatively ferocious heat radiated by nearby objects -- the sun, the Earth and even the Milky Way galaxy -- turned out to be an enormous challenge.

All that heat had to be measured or estimated, then erased from the DIRBE results.

It's a little like going to New York's Times Square one night and shutting down all the Broadway theater marquees, all the auto headlights in Manhattan, and then all the power companies in three states so you can look up and measure the light from the Milky Way.

DIRBE's detectors were insulated from COBE's heat by a tank of liquid helium that chilled them to 454 degrees below zero Fahrenheit -- approaching absolute zero.

Data analysts then had to measure, and subtract, the infrared glow from the Earth, and all the asteroids, comets and dust in the solar system -- a glow identifiable in the DIRBE data because it changed as the Earth moved around the solar system in its orbit.

They also had to erase all the bright stars mapped in our galactic neighborhood.

Finally, using computer models of the interstellar dust swirling around in the Milky Way, they were able to subtract estimates of that heat from their data. That left a picture of the sky, in all directions, with a uniform glow.